The present invention relates generally to a modular active test probe and a removable tip module therefor that optimizes test probe performance.
Test probes are particularly critical to the accurate measurement of signals. As is well known, test probes are used to make temporary connections between a circuit test point and a measuring instrument, such as an oscilloscope. The primary goal when measuring a signal is to obtain as accurate a measurement as possible without disturbing the operation of the circuit. Specifically, the goal is to sample a signal without unduly loading the circuit while maximizing signal fidelity. For purposes herein, signal fidelity refers to the accuracy with which the signal that would be present at an unloaded test point is transmitted to the measuring apparatus and is achieved by, among other things, minimizing signal attenuation, maximizing bandwidth, providing constant time delay with increasing frequency, and minimizing ringing, signal reflection, and other types of signal distortion. To design a test probe that is capable of satisfying this goal, the properties of the test probe, the probe cable, and the measuring instrument must be considered together.
It is also important when measuring signals to have a test probe that is adapted to specific measurement needs. First, different test instruments have different input requirements, e.g., bandwidth, input resistance, and input capacitance, and the test probe and probe cable used with a particular test instrument should be compatible with these requirements. Second, different test probes are adapted for sampling different test conditions. Voltage probes may be adapted for measuring high frequency or differential signals. Test probes may be adapted to the specific geometries or electrical characteristics of the circuit being tested. For example, some circuits now employ a standard connector for attaching a test probe. Moreover, test probes may be adapted to measure different types of signals, such as current or optical signals.
Test probes typically include a probe tip that makes physical contact with the test point, a probe body that allows the test probe to be held and which also holds the probe tip and probe circuit components, and a probe cable used to couple the test probe to the test instrument. An active test probe additionally includes a high input impedance amplifier to provide high signal fidelity while minimizing loading of the circuit.
Typically, the probe body and probe tip are integral. However, in some test probes, the probe tip can be removed from the probe body and replaced with another probe tip. A removable probe tip is desirable for several reasons: (a) different types of probe tips are required or may be advantageous for different test conditions; (b) the use of a single probe and several removable probe tips of different types is less expensive and more convenient than using several different complete test probe assemblies; and (c) if the removable probe tip breaks, it is less expensive to replace the probe tip rather than the complete test probe assembly. Though removable probe tips are desirable, they suffer from a number of disadvantages.
As mentioned, the design of a test probe capable of sampling a signal without unduly loading the circuit while maximizing signal fidelity requires that the test probe, probe cable, and test instrument properties be considered together. For example, the circuit elements in an active test probe are typically designed to optimize the performance of a probe tip having a particular geometry. However, these same circuit elements will not provide optimal performance when a probe tip having a different geometry is substituted. In other words, a test probe may be designed to optimize the signal fidelity for a single probe tip, but signal fidelity will not be optimal if the test probe is used with a number of different probe tips.
Other disadvantages of removable probe tips arise from the fact that removable tips require at least one (and typically more than one) removable connection or connector in the signal path. The connectors are needed to mechanically join and electrically couple the probe body and the probe tip. One common type of electrical connector is a socket on the probe body that receives the probe tip. Within the socket is a pliant spring or elastomer that compresses to receive and engage the probe tip after it has been inserted. The socket, the probe tip, and spring are all made from conductive material, such as metal. The socket is coupled to probe circuitry within the probe body and electrically couples the probe circuitry with the probe tip. In another common type of electrical connector, threaded members, such as male and female coaxial cable connectors or a threaded probe tip and socket, are used to join the probe tip and the socket together. The threaded connection generally holds the probe tip rigidly, but employs more metal than is used in pliant connectors.
The mechanical requirements for removably coupling metal parts typically increase the use of conductive materials and thereby increase the parasitics of the test probe, which degrades signal fidelity, particularly by decreasing bandwidth. As will be appreciated by one skilled in the art, the probe tip and probe body have a parasitic capacitance and inductance (“parasitics”). The amount of conductive material in the signal path is directly proportional to the magnitude of the parasitic components. In the past, test probe parasitics have not been as significant a problem as they are today. The reason is that test probe parasitics are generally not a significant problem at low frequencies. At the high frequencies (e.g., 6 GHz and higher) that are commonplace in circuits today, even a small increase in parasitic capacitance in the signal path will significantly increase the load placed on the circuit under test. In addition, at high frequencies, the effects of test probe parasitics on signal fidelity significantly increase.
Test probes that have connectors that employ a threaded connection generally have more metal than is used in pliant connectors, thus such connectors have relatively high parasitics. Test probes that have connectors that employ springs or elastomers to join mechanically the probe body with the probe tip have lower (though still high) parasitics. However, the level of parasitics found in spring or elastomer connectors have the additional problem of being variable. As the relative positions and shape of the springs as well as the position of the probe tip change in response to forces encountered by the probe tip, the amount of parasitics also varies. The surface area of the probe tip that is in contact with the socket can also change in response to these forces, changing the parasitics of the probe. It is all but impossible to optimize a test probe design to compensate for variable parasitics.
Yet another general problem with the use of removable tips is the insertion of an additional distance or electrical length that is required for the repeatably removable connector. To avoid distortion, it is especially important to minimize this distance when measuring high frequency signals for which the corresponding wavelengths are not large compared with the electrical length.
In differential test probes, there are two probe tips and two signal paths each of which couples a separate signal to one of two inputs of a differential amplifier. If the differential test probe has removable probe tips that employ spring or elastomer connectors, the connector in each tip adds parasitics that vary with pressure against the respective probe tip. This causes signal fidelity errors that differ for each of the two signal paths. Therefore, there is a signal fidelity error in the differential signal that varies as a result of the respective pressures applied at each connector. As mentioned, it is all but impossible to optimize a test probe design to compensate for variable parasitics.
Accordingly, there is a need for a modular active test probe and removable tip module therefor that optimizes test probe performance.